Somatomedin, formerly called sulfation factor, is a growth hormone dependent factor which acts directly on cartilage in vitro, whereas growth hormone does not. It causes stimulation of the incorporation of thymidine into DNA, uridine into RNA, leucine into protein, as well as sulfate into mucopolysaccharide. Inhibition of protein synthesis either by cycloheximide or indirectly by Actanomycin-D's inhibition of RNA synthesis inhibits thymidine incorporation. There is little effect of cycloheximide on uridine incorporation into RNA. Thus, it seems that RNA stimulation is a primary event. The above studies on mechanisms have involved isolated chondrocytes from pelvic rudiments of chick embryos. Results with this test system on clinical serum specimens have sometimes been at variance with other somatomedin assays. Attempts are being made to rationalize these differences. Studies are also proposed regarding the correlation of the binding of somatomedin to siolated cells and its physiological resultants.